The present invention relates to a needle thread tension control system provided with a pair of thread clamping members for checking and permitting a supply of needle thread in respective predetermined timing. The clamping members are disposed in relationship to the sewing operation in a thread supply path portion between a thread supply spool and a thread guide of a take-up lever along a thread supply path extending from the thread supply spool to an eye of a needle. A mechanism is provided for driving the thread clamping members, for controlling the driving so as to make an amount of disengaging displacement of the thread clamping members during a predetermined or unit angle of an arm shaft in releasing the needle thread smaller than an amount of engaging displacement of the thread clamping members during rotation of unit angle of the arm shaft in clamping the needle thread.
Generally, the take-up lever driven vertically in timed relation with the arm shaft supplies the needle thread toward a bobbin when the take-up lever descends and also tightens needle thread loop made at the eye of the needle and supplementing a predetermined amount of the needle thread from the thread supply spool when the take-up lever ascends.
Within the thread supply path between the thread supply spool and the thread guide of the take-up lever, there is provided generally a thread controller that provides the needle thread with passage resistance to enable tightening of the needle thread loop. This needle thread controller has a pair of thread control discs contacting each other under pressure of a compression spring and exerts a frictional resistance on the thread passing between the pair of thread control discs.
However, in the needle thread controllers of this kind, the needle thread is supplied under interaction of tension that acts on the needle thread following the thread controller and resistive force in the thread controller. Therefore, it is difficult to exactly control the force of tightening the needle thread and supply quantity of the needle thread and it is also difficult to adjust the needle thread condition of stitches in response to thicknesses and types of work fabrics (that is, thickness and type of the needle thread to be selected according to these factors).
Therefore, as disclosed in Japanese Patent Publication (examined) No. 53-41580, in place of the above described spring actuated thread controller, an electrically actuated needle thread passage control device has been proposed which comprises a pair of the thread control discs that contact under pressure of a solenoid actuator, and allows the needle thread to pass at a predetermined timing during a given time period and does not allow the thread to pass during a period except during the above predetermined timing by driving the actuator in timed relation with the arm shaft rotation.
In the needle thread passage control device described in the above publication, it can only control the thread passage in synchronism with the arm shaft rotation speed regardless of thicknesses and types of the work fabrics, that is, regardless of thicknesses of the needle thread. Further, operation speeds when this control device releases or regulates the thread passage are set independent of the arm shaft rotation speed (sewing speed).
Therefore, this control device performs the same tightening of the needle thread and supplies the same quantity of the needle thread regardless of a thick or a thin thread and can not tighten the needle thread so as to generate a tension corresponding to the thread thickness. This results in unstable thread tightening.
Further, because a supply amount of the needle thread varies slightly as the arm shaft rotation speed varies, the thread tightening varies according to the sewing speed.
To solve these problems, the applicant of the present invention proposed previously an automatic needle thread supply control system for a sewing machine (Refer to application Ser. No. 07/016,332 U.S. Pat. No. 4,791,876). This automatic needle thread supply control system comprises a take-up lever formed with an upper half portion of a swing lever driven by the arm shaft, a take-up lever driving mechanism for driving the swing lever so that the take-up lever is held at the uppermost position from a time when the arm shaft rotated by approximately 40 degrees after the needle bar arrives at its uppermost position to a time when the eye of the needle reaches the upper surface of a throat plate, and a thread passage control device installed in the thread supply path portion between the thread supply spool and the thread guide of the take-up lever along the thread supply path from the thread supply spool to the eye of the needle for checking and permitting the needle thread in timed relation with the rotation of the arm shaft.
The above described thread passage control device comprises a thread guide plate fixed on the machine frame and a clamping wheel as a pair of thread clamping members, and comprises driving means for driving the clamping wheel toward and away from the thread guide plate by means of a rotating cam on the arm shaft through a link mechanism in timed relation with the rotation of the arm shaft.
The thread passage control means is controlled so as to clamp the needle thread with the clamping wheel during tightening of the needle thread when the take-up lever ascends to its uppermost position, and to supply the needle thread of a required (consumed) amount by releasing the clamping wheel from the thread guide plate before the completion of fabric feed motion after tightening of the needle thread, and then to clamp the needle thread with the clamping wheel before the eye of the needle arrives at the throat plate.
In the thread passage control means above-described, since the releasing timing delays in accordance with increase in the thickness of the needle thread when releasing the clamping wheel, the needle thread is tightened with a tightening force corresponding to the thickness of the needle thread, and since the clamping timing is advanced in accordance with increase in the thickness of the needle thread when clamping the needle thread with holding the take-up lever at its uppermost position, a supplying amount of the needle thread decreases and thus the tightening force increases in accordance with increase in the thickness of the needle thread.
As is described above, the tightening force is controlled in accordance with the thickness of the needle thread, both in releasing control and clamping control.
In the automatic needle thread control system, releasing and clamping characteristics are determined with an elliptic locus of an elliptic cam groove formed in the rotating cam on the arm shaft. Therefore, a timing when the clamping wheel separates to a maximum extent from the thread guide plate is in the middle of a releasing timing of releasing the thread and a clamping timing of clamping the thread, and releasing characteristics and clamping characteristics are symmetrical with each other. As a result, disengaging displacement of the clamping wheel during rotation of a predetermined unit angle of the arm shaft in releasing the needle thread is comparatively large, whereby it is difficult to make a high sensitivity of releasing timing varying in accordance with a variation of the thickness of the thread.
Furthermore, since the thread tightening force is controlled at two stages of releasing control and clamping control in accordance with the thread thickness, too strong thread tightening force causes snapping of the needle thread and damaging of the needle.